1. Technical Field
The present invention relates to a solid state imaging device with a high quality image property and low power consumption.
2. Related Art
As solid state imaging devices mounted in cellular phones, digital cameras and the like, there are a charge coupled device (CCD) type image sensor and a complementary metal-oxide semiconductor (CMOS) type image sensor.
In recent years, a MOS type solid state imaging device (hereinafter called “substrate modulation type sensor”) employing a threshold voltage modulation method has been proposed. Japanese Unexamined Patent Publication No. 2002-134729 is a first example of related art. The substrate modulation type sensor has a high quality image property and a low power consumption property. Such substrate modulation type sensor is disclosed in the first example.
The CCD sensor has a correlated double sampling (CDS) function and a so-called collective electronic shutter function for taking a fast moving object without making its image distort. In the collective electronic shutter function, a plurality of light receiving elements arranged in a matrix in plane simultaneously store electric charges that are generated by light. In this way, it can prevent the image of the moving object from being distorted. Accordingly, the CCD sensors generally have the advantage of a good image quality. However, the CCD sensors also have disadvantages of a high driving voltage and high power consumption.
On the other hand, the CMOS sensors generally consume less power and their process cost is low. However, it is impossible for the common CMOS sensors to realize both the collective electronic shutter function and the CDS function.
To explain this, an operation of the CMOS sensor is briefly described, taking a CMOS-APS (active pixel sensor) type that has four transistors as an example. Firstly, a floating diffusion, which is an electric charge storage area, is reset. Secondly, an electric potential based on the photo-generated electric charges remaining in the floating diffusion is outputted (readout of a noise component). After that, photo-generated electric charges generated by a photodiode are transferred to the floating diffusion. An electric potential based on the photo-generated electric charges transferred to the floating diffusion is then outputted (readout of a signal component). The CDS function is realized by taking a difference between these electric potential.
In the CDS, the readout of the signal has to be carried out right after the readout of the noise. As for the hitherto known CMOS sensors, the readout operation is performed with respect to each line so that the photo-generated electric charges have to be transferred to the floating diffusion line by line in order to realize the CDS. On the other hand, in order to realize the collective electronic shutter, the photo-generated electric charges in the all the pixels have to be transferred to the floating diffusion all at once. In other words, the transferring method of the photo-generated electric charges differs depending on the collective electronic shutter function or the CDS function.
When the photo-generated electric charges in all the pixels are transferred to the floating diffusion all at once in order to realize the collective electronic shutter function, the floating diffusion cannot be reset before the noise readout because the photo-generated electric charges are already stored in the floating diffusion. This means that the collective electronic shutter function and the CDS function cannot be realized at the same time.
Japanese Unexamined Patent Publication No. 2002-368201 is a second example of related art. In order to solve the above-mentioned problem, the second example discloses a technique that timing of the start and finish of a signal accumulation operation is synchronized in all the pixels. According to the second example, a charge retention region is provided right under a transfer gate, and a signal charge from the photodiode is once stored in the charge retention region and then transferred to the floating diffusion. In this way, the second example realizes the collective electronic shutter function.
However, in the device described in the second example, an N-type semiconductor region which serves as a signal charge retention region is formed on a semiconductor substrate surface. More specifically, the N type semiconductor region is formed so as to contact with a gate insulating layer. In this regard, there is a problem that electric charges tend to be generated and trapped in this contact area, and dark current tends to be generated.
Moreover, there is a possibility that a potential distribution in which some electric charges are left without being transferred is formed in a sectional structure disclosed in the second example.